Band-pass filter and image display apparatus

ABSTRACT

A band-pass filter has a reflecting optical element having a fixed reflection angle selection region for wavelengths of a given region, and has an optical path such that reflection is performed at least once and transmission is performed at least once at the reflecting optical element.

RELATED APPLICATION

[0001] This application is based on application No. 2001-359069 filed inJapan, the content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a band-pass filter and an imagedisplay apparatus, and more specifically, to an image display apparatussuch as a head mounted display (HMD) projecting two-dimensional imageson a display device such as a liquid crystal display (LCD) onto theviewer's eye through an eyepiece optical system, and a band-pass filterand an illuminating optical system used for the image display apparatus.

DESCRIPTION OF THE PRIOR ART

[0003] Light of a wavelength distribution necessary for the imagedisplay apparatus is obtained by using a band-pass filter. However, itis difficult to achieve size and weight reduction of the band-passfilter while securing a sharp wavelength distribution. Opticalstructures using the angle selectivity of a holographic optical element(HOE) to achieve optical element size reduction are proposed in U.S.Pat. Nos. 3,940,203, 4,830,464 and 4,874,214 and European PatentApplication No. EP 943,934. The optical structures proposed by thesedocuments adopt a so-called “pancake” system in which transmission andreflection are repeated between two opposed optical elements.

[0004] In these conventional examples, the diffraction action of the HOEis used for the power of the eyepiece optical system while the opticalelement thickness is reduced by bending the optical path. For thisreason, the wavelength distribution of light is never changed when thelight is reflected and transmitted, so that no band-pass filter effectis obtained although size reduction is achieved. In addition, theconventional examples cannot be used for illumination and beam shapingnecessary for the image display apparatus.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide an improvedband-pass filter and an image display apparatus using the band-passfilter.

[0006] Another object of the present invention is to provide a compactand lightweight band-pass filter with which a sharp wavelengthdistribution is obtained, and an image display apparatus capable ofdisplaying high-quality images by using the band-pass filter.

[0007] Still another object of the present invention is to provide acompact illuminating optical system and a compact beam shaping opticalsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] This and other objects and features of this invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanied drawings inwhich:

[0009]FIG. 1 is an optical structure view showing the schematicstructure of a first embodiment;

[0010] FIGS. 2(A) and 2(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe first embodiment;

[0011]FIG. 3 is an optical structure view showing the schematicstructure of a second embodiment;

[0012]FIG. 4 is an optical structure view showing the schematicstructure of a third embodiment;

[0013]FIG. 5 is a cross-sectional view showing an example of areflecting optical element used in the first to the third embodiments;

[0014]FIG. 6 is an optical structure view showing the schematicstructure of a fourth embodiment;

[0015]FIG. 7 is an optical structure view showing the schematicstructure of a fifth embodiment;

[0016]FIG. 8 is an optical structure view showing the schematicstructure of a sixth embodiment;

[0017] FIGS. 9(A) and 9(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe sixth embodiment;

[0018]FIG. 10 is an optical structure view showing the schematicstructure of a seventh embodiment;

[0019]FIG. 11 is an optical structure view showing the schematicstructure of an eighth embodiment;

[0020]FIG. 12 is an optical structure view showing the schematicstructure of a ninth embodiment;

[0021]FIG. 13 is an optical structure view showing the schematicstructure of a tenth embodiment;

[0022]FIG. 14 is an optical structure view showing the schematicstructure of an eleventh embodiment;

[0023]FIG. 15 is an optical structure view showing the schematicstructure of a twelfth embodiment;

[0024] FIGS. 16(A) and 16(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe twelfth embodiment;

[0025]FIG. 17 is an optical structure view showing the schematicstructure of a thirteenth embodiment;

[0026]FIG. 18 is an optical structure view showing the schematicstructure of a fourteenth embodiment;

[0027]FIG. 19 is an optical structure view showing the schematicstructure of a fifteenth embodiment;

[0028] FIGS. 20(A) and 20(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe fifteenth embodiment;

[0029]FIG. 21 is an optical structure view showing the schematicstructure of a sixteenth embodiment;

[0030] FIGS. 22(A) and 22(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe sixteenth embodiment;

[0031]FIG. 23 is an optical structure view showing the schematicstructure of a seventeenth embodiment;

[0032] FIGS. 24(A) and 24(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe seventeenth embodiment;

[0033]FIG. 25 is an optical structure view showing the schematicstructure of an eighteenth embodiment;

[0034] FIGS. 26(A) and 26(B) are graphs showing the wavelengthdistributions of light beams before and after wavelength limitation inthe eighteenth embodiment;

[0035]FIG. 27 is an optical structure view showing the schematicstructure of a nineteenth embodiment;

[0036]FIG. 28 is an optical structure view showing the schematicstructure of a twentieth embodiment;

[0037]FIG. 29 is an optical structure view showing the schematicstructure of a twenty-first embodiment;

[0038]FIG. 30 is an optical structure view showing the schematicstructure of a twenty-second embodiment;

[0039]FIG. 31 is an optical structure view showing the schematicstructure of a twenty-third embodiment;

[0040]FIG. 32 is an optical structure view showing the schematicstructure of a twenty-fourth embodiment;

[0041]FIG. 33 is an optical structure view showing the schematiccross-sectional structure, in an x direction, of a twenty-fifthembodiment;

[0042]FIG. 34 is an optical structure view showing the schematiccross-sectional structure, in a y direction, of the twenty-fifthembodiment;

[0043] FIGS. 35(A) and 35(B) are schematic views of assistance inexplaining beam shapes before and after shaping in the twenty-fifthembodiment;

[0044]FIG. 36 is an optical structure view showing the schematicstructure of a twenty-sixth embodiment;

[0045]FIG. 37 is an optical structure view showing the schematicstructure of a twenty-seventh embodiment;

[0046]FIG. 38 is an optical structure view showing the schematicstructure of a twenty-eighth embodiment;

[0047]FIG. 39 is an optical structure view showing the schematicstructure of a twenty-ninth embodiment;

[0048]FIG. 40 is an optical structure view showing the schematicstructure of a thirtieth embodiment; and

[0049]FIG. 41 is a perspective view showing the schematic externalstructure where any of the embodiments of the present invention isapplied to an eyeglass-type image display apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0050] Hereinafter, a band-pass filter, an image display apparatus andthe like embodying the present invention will be described withreference to the drawings. The same and corresponding parts of theembodiments are designated by the same reference numbers and overlappingdescriptions are omitted as appropriate.

[0051] <<Embodiments of the Band-Pass Filter (FIGS. 1 to 7); the Numberof Reflecting Optical Elements is One, and Wavelength Limitation isPerformed at the Time of Transmission or Reflection>>

[0052]FIG. 1 shows the optical structure and the optical path of a firstembodiment. The first embodiment is a band-pass filter having aflat-shaped reflecting optical element (D) and a mirror (M)substantially parallelly opposed to the reflecting optical element (D).The reflecting optical element (D) has a fixed reflection angleselection region for wavelengths of a given region, and is capable ofreflecting and transmitting light in accordance with the incident angleof the light by its angle selectivity.

[0053] As the reflecting optical element (D), a volume-phase andreflecting holographic optical element (HOE) is used. As the materialtherefor, although it is desirable to use photopolymer capable of beingformed by a dry process, a silver salt material, gelatin dichromate orthe like may be used as well. The reflecting optical element (D) is notlimited to the HOE and other diffraction optical elements. It may be,for example, a multilayer film or a multilayer filter (see U.S. Pat. No.6,157,490). The mirror (M) is a normal mirror that reflects lightwithout affecting the wavelength distribution of the light. While thereflecting optical element (D) and the mirror (M) are opposed to eachother with a predetermined distance in between, they may be bondedtogether as long as the incidence of a light beam (L0) on the reflectingoptical element (D) is not hindered. By bonding the reflecting opticalelement (D) and the mirror (M) together, the band-pass filter can befurther reduced in thickness.

[0054] In the band-pass filter shown in FIG. 1, the optical path is setso that the incident light beam (L0) from a light source such as a lightemitting diode (LED) is obliquely incident on the reflecting opticalelement (D) and substantially vertically reflected because of thediffraction action there, the reflected light beam (L1) is reflected bythe mirror (M), and the reflected light beam (L2) is substantiallyvertically transmitted by the reflecting optical element (D) and exitsas a transmitted light beam (L3). At the reflecting optical element (D),reflection and transmission are each performed once, and at that time,the incident light beam (L0) and the reflected light beam (L2) areincident on the reflecting optical element (D) at different angles.Since the reflecting optical element (D) has a fixed reflection angleselection region for wavelengths of a given region, a wavelength regionon the longer wavelength side or the shorter wavelength side of thewavelength distribution of the reflected light beams (L1, L2) obtainedby the reflection at the reflecting optical element (D) can be cut awayby the transmission at the reflecting optical element (D). An examplethereof is shown in FIGS. 2(A) and 2(B).

[0055] The graphs in FIGS. 2(A) and 2(B) show the wavelengthdistributions of the light beams before and after the wavelengthlimitation on the longer wavelength side (the horizontal axis representsthe wavelength, and the vertical axis represents the light intensity).As shown in FIG. 2(A), first, the wavelength distribution of theincident light beam (L0) becomes one with a narrow wavelength width (L1,L2) because of the reflection at the reflecting optical element (D).Then, a wavelength region (E, the hatched part on the right side of thebroken line) on the longer wavelength side of the wavelengthdistribution of the reflected light beam (L2) is cut away by thetransmission at the reflecting optical element (D), so that thewavelength width is narrower. Consequently, as shown in FIG. 2(B), asharp wavelength distribution (L3) with a narrow wavelength width isobtained.

[0056] By performing reflection at least once and transmission at leastonce at different incident angles at the reflecting optical element (D)having angle selectivity for given wavelengths as described above, theband-pass filter can be reduced in thickness and wavelength limitationto arbitrarily change the wavelength distribution of the incident lightbeam (L0) can be performed. In that case, by performing at least oncethe transmission or the reflection to cut away a wavelength region onthe longer wavelength side or the shorter wavelength side of thewavelength distribution obtained by performing reflection ortransmission at least once, the wavelength distribution of the incidentlight beam (L0) can be changed to one with a narrow width (L3).Consequently, by appropriately controlling the angle of incidence on thereflecting optical element (D), the band-pass filter can be reduced insize and weight, and further, a sharp wavelength distribution can beeasily obtained. From this point of view, the order in which the actionsare performed on the light at the reflecting optical element (D) is notlimited to reflection first and transmission second, but may betransmission first and reflection second. A second to a fifth embodimentthus structured will be described below.

[0057]FIGS. 3 and 4 show the optical structures and the optical paths ofthe second and the third embodiments. The second embodiment (FIG. 3) isa band-pass filter having a flat-shaped reflecting optical element (D)and a half mirror (HM) disposed obliquely to the reflecting opticalelement (D). The third embodiment (FIG. 4) is a band-pass filter havinga spherical reflecting optical element (D) and a half mirror (HM)disposed obliquely to the reflecting optical element (D). The opticalsurface constituting the reflecting optical element (D) may be a planesurface like those of the first and the second embodiment or may be acurved surface like that of the third embodiment. By using a curvedsurface for the reflecting optical element (D), an optical power can beeasily provided. However, when the reflecting optical element (D)comprises an HOE, an optical power can be provided even when its opticalsurface is a plane surface. The cross-sectional view of FIG. 5 shows anexample of the reflecting optical element (D) comprising an HOE used forthe first to the third embodiments. In FIG. 5, d1 represents an antireflection (AR) coating, d2 represents a glass substrate, d3 representsan HOE layer, and d4 represents a barrier layer.

[0058] In the band-pass filters shown in FIGS. 3 and 4, the optical pathis set so that the incident light beam (L0) is transmitted by thereflecting optical element (D), the transmitted light beam (L1) isreflected by the half mirror (HM), the reflected light beam (L2) isreflected because of the diffraction action at the reflecting opticalelement (D), and the reflected light beam (L3) is transmitted by thehalf mirror (HM) and exits as a transmitted light beam (L4). Like in thefirst embodiment, reflection and transmission are each performed once atthe reflecting optical element (D), and at that time, the incident lightbeam (L0) and the reflected light beam (L2) are incident on thereflecting optical element (D) at different angles. Since the reflectingoptical element (D) has a fixed reflection angle selection region forwavelengths of a given region, a wavelength region on the longerwavelength side or the shorter wavelength side of the wavelengthdistribution of the light beams (L1, L2) obtained by the transmission atthe reflecting optical element (D) can be cut away by the reflection atthe reflecting optical element (D).

[0059] When the order in which the actions are performed on the light atthe reflecting optical element (D) is transmission first and reflectionsecond, it is easy to use a selectively reflecting mirror such as thehalf mirror (HM) having light quantity selectivity in combination withthe reflecting optical element (D). Examples of the selectivelyreflecting mirror include, in addition to the half mirror (HM), apolarizing beam splitter (PBS), a sheet-shaped beam splitter (forexample, the product name, DBEF, manufactured by Sumitomo 3M Ltd.), awire grid and cholesteric liquid crystal. The optical structures and theoptical paths of the fourth and the fifth embodiments having aselectively reflecting mirror other than the half mirror (HM) are shownin FIGS. 6 and 7.

[0060] The fourth embodiment (FIG. 6) is a band-pass filter having aflat-shaped reflecting optical element (D), and a structure comprising aquarter-wave plate (W) and a sheet-shaped beam splitter (P1) bondedtogether which structure is substantially parallelly opposed to thereflecting optical element (D). The fifth embodiment (FIG. 7) is aband-pass filter having a flat-shaped reflecting optical element (D),and a structure comprising a quarter-wave plate (W) and cholestericliquid crystal (P2) bonded together which structure is substantiallyparallelly opposed to the reflecting optical element (D). Polarizingoptical elements such as the sheet-shaped beam splitter (P1) and thecholesteric liquid crystal (P2) are reflecting members havingpolarization selectivity (that is, a property of reflecting only aspecific linearly polarized light beam and transmitting the other kindsof light beams). The PBS and the wire grid are also reflecting membershaving this property.

[0061] In the band-pass filters shown in FIGS. 6 and 7, a circularlypolarized light beam from a display device such as an LCD is transmittedby the reflecting optical element (D) as an incident light beam (L0),and the transmitted light beam (L1) becomes a linearly polarized lightbeam (for example, an s-polarized light beam) by being transmitted bythe quarter-wave plate (W), is reflected by the polarizing opticalelements (P1, P2) and then, becomes a circularly polarized reflectedlight beam (L2) by being again transmitted by the quarter-wave plate(W). The reflected light beam (L2) is reflected because of thediffraction action at the reflecting optical element (D), and thereflected light beam (L3) becomes a linearly polarized light beam (forexample, a p-polarized light beam) by being transmitted by thequarter-wave plate (W). Then, the linearly polarized light beam istransmitted by the polarizing optical elements (P1, P2) and exits as atransmitted light beam (L4). By structuring the band-pass filter so thatpolarized light is made incident on the reflecting optical element (D)by using the polarizing optical elements (P1, P2) as described above,unnecessary light such as ghost can be cut.

[0062] <<Embodiments of the Band-Pass Filter (FIGS. 8 to 14); the Numberof Reflecting Optical Elements is Two or Three, and WavelengthLimitation is Performed at the Time of Transmission>>

[0063]FIG. 8 shows the optical structure and the optical path of a sixthembodiment. The sixth embodiment is a band-pass filter where twoflat-shaped first and second reflecting optical elements (D1, D2) aresubstantially parallelly opposed. The first and the second reflectingoptical elements (D1, D2) each have a fixed reflection angle selectionregion for wavelengths of a given region like the reflecting opticalelement (D), and are capable of reflecting and transmitting light inaccordance with the incident angle of the light by their angleselectivities for the given wavelengths (HOEs, multilayer films,multilayer filters or the like). However, the reflection angle selectionregions of the first and the second reflecting optical elements (D1, D2)are different from each other. The difference in angle selectivityenables the first and the second reflecting optical elements (D1, D2) tobe substantially parallelly opposed, and this enables a furtherthickness reduction of the band-pass filter. Changing the optical pathenables the use of a first and a second reflecting optical element (D1,D2) having the same reflection angle selection region.

[0064] In the band-pass filter shown in FIG. 8, the optical path is setso that the incident light beam (L0) from a light source such as an LEDis obliquely transmitted by the first reflecting optical element (D1),the transmitted light beam (L1) is obliquely reflected by the secondreflecting optical element (D2), the reflected light beam (L2) isreflected by the first reflecting optical element (D1), and thereflected light beam (L3) is substantially vertically transmitted by thesecond reflecting optical element (D2) and exits as a transmitted lightbeam (L4). At the first and the second reflecting optical elements (D1,D2), reflection and transmission are each performed once. At that time,the incident light beam (L0) and the reflected light beam (L2) areincident on the first reflecting optical element (D1) at differentangles, and the transmitted light beam (L1) and the reflected light beam(L3) are incident on the second reflecting optical element (D2) atdifferent angles. Since the second reflecting optical element (D2) has afixed reflection angle selection region for wavelengths of a givenregion, a wavelength region on the longer wavelength side or the shorterwavelength side of the wavelength distribution of the reflected lightbeam (L3) obtained by the transmission at the first reflecting opticalelement (D1) and the reflection at the first and the second reflectingoptical elements (D1, D2) can be cut away by the transmission at thesecond reflecting optical element (D2). An example thereof is shown inFIGS. 9(A) and 9(B). The graphs in FIGS. 9(A) and 9(B) show thewavelength distributions of the light beams before and after thewavelength limitation on the longer wavelength side (the horizontal axisrepresents the wavelength, and the vertical axis represents the lightintensity). As shown in FIG. 9(A), a wavelength region (E, the hatchedpart on the right side of the broken line) on the longer wavelength sideof the wavelength distribution of the reflected light beam (L3) is cutaway by the transmission at the second reflecting optical element (D2),so that the wavelength width is narrower. Consequently, as shown in FIG.9(B), a sharp wavelength distribution (L4) with a narrow wavelengthwidth is obtained. Since two reflecting optical elements (D1, D2) areused, a band-pass effect with a narrower wavelength width can beobtained.

[0065] By performing transmission and reflection at the first reflectingoptical element (D1) and reflection at the second reflecting opticalelement (D2) as described above, the wavelength width of the wavelengthdistribution of the incident light beam (L0) can be made narrow (L3),and further, by performing transmission to limit the wavelength of thereflected light beam (L3) at the second reflecting optical element (D2),the wavelength distribution of the incident light beam (L0) can bearbitrarily changed. That is, in the sixth embodiment, the degree offreedom of the wavelength limitation is increased by using the firstreflecting optical element (D1) instead of the mirror (M) used in thefirst embodiment (FIG. 1). Therefore, by performing transmission atleast once and reflection at least once at the first and the secondreflecting optical elements (D1, D2) to reduce the thickness of theband-pass filter, the wavelength distribution of the incident light beam(L0) can be arbitrarily changed by performing another transmission tolimit the wavelength of the light beam before and after the transmissionand the reflection.

[0066] In that case, by performing at least once the transmission to cutaway a wavelength region on the longer wavelength side or the shorterwavelength side of the wavelength distribution obtained by performingreflection or transmission at least once, the wavelength distribution ofthe incident light beam (L0) can be changed to one with a narrow width(L4). Consequently, by appropriately controlling the angles of incidenceon the first and the second reflecting optical elements (D1, D2), theband-pass filter can be reduced in size and weight, and further, a sharpwavelength distribution can be easily obtained. From this point of view,the disposition, the optical path, the number and the like of thereflective optical elements (D1, D2) are not limited to the ones of thesixth embodiment (FIG. 8). A seventh to an eleventh embodiment havingsuch a structure will be described below.

[0067] FIGS. 10 to 14 show the optical structures and the optical pathsof the seventh to the eleventh embodiments. In the seventh embodiment(FIG. 10), the actions on light and the optical paths of the first andthe second reflecting optical elements (D1, D2) in the sixth embodiment(FIG. 8) are interchanged. Therefore, the incident light beam (L0) issubstantially vertically incident on the first reflecting opticalelement (D1), and the transmitted light beam (L4) exits obliquely to thesecond reflecting optical element (D2). In the eighth and the ninthembodiments (FIGS. 11 and 12), the first and the second reflectingoptical elements (D1, D2) are non-parallelly opposed. By thus disposingthe reflecting optical elements (D1, D2) so as to be inclined withrespect to each other, the reflecting optical elements (D1, D2) canreflect or transmit a light beam of a different wavelength region evenwhen their angle selectivities are the same. Consequently, band-passfilters reflecting or transmitting light beams of different wavelengthregions can be easily formed. Moreover, by disposing the reflectingoptical elements (D1, D2) so as to be inclined with respect to eachother, an advantage is obtained that the diffraction efficiencies of theHOEs constituting the reflecting optical elements (D1, D2) can beimproved. However, when the first and the second reflecting opticalelements (D1, D2) comprise HOEs, an effect produced by disposing thereflecting optical elements so as to be inclined with respect to eachother can be easily provided also when the reflecting optical elementsare parallelly opposed.

[0068] The tenth embodiment (FIG. 13) is a band-pass filter comprisingthree first to third reflecting optical elements (D1, D2, D3). By thuscombining at least three reflecting optical elements (D1, . . . ), alarger band-pass effect can be obtained. Moreover, addition of aplurality of functions such as provision of an optical power ispossible. In the ninth and the tenth embodiments (FIGS. 12 and 13),reflection at the first and the second reflecting optical elements (D1,D2) and at the third reflecting optical element (D3) is performed atotal of four times. Further, in the tenth embodiment (FIG. 13),transmission at the first to the third reflecting optical elements (D1to D3) is performed a total of five times. By thus performing reflectionand transmission at the reflecting optical elements (D1, . . . ) atleast three times, wavelength cutting can be stepwisely performed bygradually changing the angles of incidence on the reflecting opticalelements (D1, . . . ). Consequently, a sharper wavelength distributioncan be obtained.

[0069] In the eleventh embodiment (FIG. 14), the first and the secondreflecting optical elements (D1, D2) are non-parallelly opposed like inthe eighth and the ninth embodiments (FIGS. 11 and 12). At the firstreflecting optical element (D1), only one reflection is performed, andat the second reflecting optical element (D2), only one transmission isperformed. The incident light beam (L0) is reflected by the firstreflecting optical element (D1), and wavelength limitation is performedby the reflected light beam (L1) being transmitted by the secondreflecting optical element (D2). Consequently, the wavelengthdistribution of the incident light beam (L0) is changed to one with anarrow wavelength width (L2), so that a sharp wavelength distribution isobtained.

[0070] <<Embodiments of the Band-Pass Filter (FIGS. 15 to 18); theNumber of Reflecting Optical Elements is Two, and Wavelength Limitationis Performed at the Time of Reflection>>

[0071]FIG. 15 shows the optical structure and the optical path of atwelfth embodiment. The twelfth embodiment is characterized in that thewavelength limitation is performed when light is reflected by the firstreflecting optical element (D1). Except this, the structure is similarto that of the sixth embodiment (FIG. 8). Since the first reflectingoptical element (D1) has a fixed reflection angle selection region forwavelengths of a given region, a wavelength region on the longerwavelength side or the shorter wavelength side of the wavelengthdistribution of the reflected light beam (L2) obtained by thetransmission at the first reflecting optical element (D1) and thereflection at the second reflecting optical element (D2) can be cut awayby the reflection at the first reflecting optical element (D1). Anexample thereof is shown in FIGS. 16(A) and 16(B).

[0072] The graphs in FIGS. 16(A) and 16(B) show the wavelengthdistributions of the light beams before and after the wavelengthlimitation on the shorter wavelength side (the horizontal axisrepresents the wavelength, and the vertical axis represents the lightintensity). As shown in FIG. 16(A), since the wavelength distribution(L3′) of the light beam reflected by the first reflecting opticalelement (D1) is on the longer wavelength side of the wavelengthdistribution of the reflected light beam (L2), a wavelength region onthe shorter wavelength side of the wavelength distribution of thereflected light beam (L2) is cut away by the reflection at the firstreflecting optical element (D1), so that the wavelength width isnarrower. Consequently, a sharp wavelength distribution (L3, L4) with anarrow wavelength width is obtained as shown in FIG. 16(B). Since thesecond reflecting optical element (D2) has angle selectivity to transmitall of the wavelength distribution of the vertically incident reflectedlight beam (L3), the wavelength distributions of the reflected lightbeam (L3) and the transmitted light beam (L4) are the same.

[0073] By performing transmission at the first reflecting opticalelement (D1) and reflection at the second reflecting optical element(D2) as described above, the wavelength width of the wavelengthdistribution of the incident light beam (L0) can be made narrow (L2),and further, by performing reflection to limit the wavelength of thereflected light beam (L2) at the first reflecting optical element (D1),the wavelength distribution of the incident light beam (L0) can bearbitrarily changed. That is, in the twelfth embodiment, the degree offreedom of the wavelength limitation is increased by using the secondreflecting optical element (D2) instead of the selectively reflectingmirror (for example, the half mirror (HM)) used in the second to thefifth embodiments (FIGS. 3, 4, 6 and 7). Therefore, by performingtransmission at least once and reflection at least once at the first andthe second reflecting optical elements (D1, D2) to reduce the thicknessof the band-pass filter, the wavelength distribution of the incidentlight beam (L0) can be arbitrarily changed by performing anotherreflection to limit the wavelength of the light beam before and afterthe transmission and the reflection.

[0074] In that case, by performing at least once the reflection to cutaway a wavelength region on the longer wavelength side or the shorterwavelength side of the wavelength distribution obtained by performingreflection or transmission at least once, the wavelength distribution ofthe incident light beam (L0) can be changed to one with a narrow width(L3, L4). Consequently, by appropriately controlling the angles ofincidence on the first and the second reflecting optical elements (D1,D2), the band-pass filter can be reduced in size and weight, andfurther, a sharp wavelength distribution can be easily obtained. Fromthis point of view, the disposition and the like of the reflectiveoptical elements (D1, D2) are not limited to the ones of the twelfthembodiment (FIG. 15). A thirteenth and a fourteenth embodiment havingsuch a structure will be described below.

[0075]FIGS. 17 and 18 show the optical structures and the optical pathsof the thirteenth and the fourteenth embodiments. In these embodiments,the first and the second reflecting optical elements (D1, D2) arenon-parallelly opposed. In the thirteenth embodiment (FIG. 17), at thefirst reflecting optical element (D1), transmission and reflection areeach performed once, and at the second reflecting optical element (D2),only one reflection is performed. The incident light beam (L0) istransmitted by the first reflecting optical element (D1), and thetransmitted light beam (L1) is reflected by the second reflectingoptical element (D2). Then, the reflected light beam (L2) incident at anangle different from the angle at which the incident light beam (L0) istransmitted by the first reflecting optical element (D1) is reflected bythe first reflecting optical element (D1) to thereby perform wavelengthlimitation. Consequently, the wavelength distribution of the incidentlight beam (L0) is changed to one with a narrow wavelength width (L3),so that a sharp wavelength distribution is obtained. In the fourteenthembodiment (FIG. 18), only one reflection is performed at each of thefirst and the second reflecting optical elements (D1, D2). The incidentlight beam (L0) is reflected by the first reflecting optical element(D1), and the reflected light beam (L1) is reflected by the secondreflecting optical element (D2) to thereby perform wavelengthlimitation. Consequently, the wavelength distribution of the incidentlight beam (L0) is changed to one with a narrow wavelength width (L3),so that a sharp wavelength distribution is obtained.

[0076] <<Embodiments of the Band-Pass Filter (FIGS. 19 to 22); theNumber of Reflecting Optical Elements is Two, and Wavelength Limitationis Performed at the Time of Reflection and Transmission>>

[0077]FIG. 19 shows the optical structure and the optical path of afifteenth embodiment. The fifteenth embodiment is characterized in thatthe wavelength limitation is performed when light is reflected by thefirst reflecting optical element (D1) and when light is transmitted bythe second reflecting optical element (D2). Except this, the structureis similar to that of the sixth and the twelfth embodiments (FIGS. 8 and15). Since the first and the second reflecting optical elements (D1, D2)each have angle selectivity for given wavelengths as mentioned above, awavelength region on the longer wavelength side or the shorterwavelength side of the wavelength distribution of the reflected lightbeam (L2) can be cut away by the reflection at the first reflectingoptical element (D1), and a wavelength region on the longer wavelengthside or the shorter wavelength side of the wavelength distribution ofthe reflected light beam (L3) can be cut away by the transmission at thesecond reflecting optical element (D2). An example thereof is shown inFIGS. 20(A) and 20(B).

[0078] The graphs in FIGS. 20(A) and 20(B) show the wavelengthdistributions of the light beams before and after the wavelengthlimitation on both sides (the longer wavelength side and the shorterwavelength side) of the wavelength distribution (the horizontal axisrepresents the wavelength, and the vertical axis represents the lightintensity). As shown in FIG. 20(A), since the wavelength distribution(L3′) of the light beam reflected by the first reflecting opticalelement (D1) is on the longer wavelength side of the wavelengthdistribution of the reflected light beam (L2), a wavelength region onthe shorter wavelength side of the wavelength distribution of thereflected light beam (L2) is cut away by the reflection at the firstreflecting optical element (D1), so that the wavelength width isnarrower. Then, a wavelength region (E, the hatched part on the rightside of the broken line) on the longer wavelength side of the wavelengthdistribution of the reflected light beam (L3) is cut away by thetransmission at the second reflecting optical element (D2), so that thewavelength width is narrower. Consequently, as shown in FIG. 20(B), asharp wavelength distribution (L4) with a narrow wavelength width whereboth sides of the wavelength distribution are cut away is obtained.

[0079] By performing transmission at the first reflecting opticalelement (D1) and reflection at the second reflecting optical element(D2) as described above, the wavelength width of the wavelengthdistribution of the incident light beam (L0) can be made narrow (L2),and further, by performing reflection and transmission to limit thewavelength of the reflected light beam (L2) at the first and the secondreflecting optical elements (D1, D2), the wavelength distribution of theincident light beam (L0) can be arbitrarily changed. That is, in thefifteenth embodiment, the number of times of the wavelength limitationis increased by providing the characteristics of both of the sixth andthe twelfth embodiments (FIGS. 8 and 15). Therefore, by performingtransmission at least once and reflection at least once at the first andthe second reflecting optical elements (D1, D2) to reduce the thicknessof the band-pass filter, the wavelength distribution of the incidentlight beam (L0) can be arbitrarily changed by performing anotherreflection and transmission to limit the wavelength of the light beambefore and after the transmission and the reflection.

[0080] In that case, by performing at least once the reflection to cutaway a wavelength region on the longer wavelength side and the shorterwavelength side of the wavelength distribution obtained by performingreflection or transmission at least once, the wavelength distribution ofthe incident light beam (L0) can be changed to one with a narrow width(L4). Consequently, by appropriately controlling the angles of incidenceon the first and the second reflecting optical elements (D1, D2), theband-pass filter can be reduced in size and weight, and further, a sharpwavelength distribution can be easily obtained. From this point of view,a wavelength limitation function can be further added to the fifteenthembodiment (FIG. 19). A sixteenth embodiment having such a structurewill be described below.

[0081]FIG. 21 shows the optical structure and the optical path of thesixteenth embodiment. The sixteenth embodiment is characterized in thatthe wavelength limitation is performed when light is transmitted by thefirst reflecting optical element (D1) as well as when light is reflectedby the first reflecting optical element (D1) and when light istransmitted by the second reflecting optical element (D2). Except this,the structure is similar to that of the fifteenth embodiment (FIG. 19).Since the first reflecting optical element (D1) has a fixed reflectionangle selection region for wavelengths of a given region, a wavelengthregion on the longer wavelength side or the shorter wavelength side ofthe wavelength distribution of the incident light beam (L0) can be cutaway by the transmission at the first reflecting optical element (D1).An example thereof is shown in FIGS. 22(A) and 22(B).

[0082] The graphs in FIGS. 22(A) and 22(B) show the wavelengthdistributions of the light beams before and after the wavelengthlimitation on both sides (the longer wavelength side and the shorterwavelength side) of the wavelength distribution (the horizontal axisrepresents the wavelength, and the vertical axis represents the lightintensity). As shown in FIG. 22(A), a wavelength region (E1, the hatchedpart on the left side of the broken line) on the shorter wavelength sideof the wavelength distribution of the incident light beam (L0) is cutaway by the transmission at the first reflecting optical element (D1),so that the wavelength width is narrower. Since the wavelengthdistribution (L3′) of the light beam reflected by the first reflectingoptical element (D1) is on the longer wavelength side of the wavelengthdistribution of the light beam (L2′) reflected by the second reflectingoptical element (D2), a wavelength region on the shorter wavelength sideof the wavelength distribution of the reflected light beam (L2) is cutaway by the reflection at the first reflecting optical element (D1), sothat the wavelength width is narrower. Then, a wavelength region (E2,the hatched part on the right side of the broken line) on the longerwavelength side of the wavelength distribution of the reflected lightbeam (L3) is cut away by the transmission at the second reflectingoptical element (D2), so that the wavelength width is narrower.Consequently, a sharp wavelength distribution (L4) with a narrowerwavelength width where both sides of the wavelength distribution are cutaway is obtained as shown in FIG. 22(B).

[0083] By increasing the number of times of the wavelength limitation byrepeating reflection and transmission a plurality of times like in thesixteenth embodiment, a light beam with a wavelength distribution havinga narrow half width is obtained. The wavelengths cut away by thereflection at the first reflecting optical element (D1) and thewavelengths cut away by the reflection at the second reflecting opticalelement (D2) may be the same. That is, the wavelength widths of thewavelength distributions of the reflected light beam (L2) and thereflected light beam (L3) may be the same. Therefore, wavelengthlimitation may be performed at the time of the reflection at the secondreflecting optical element (D2) instead of at the time of the reflectionat the first reflecting optical element (D1).

[0084] <<Embodiment of the Band-Pass Filter (FIGS. 23 and 24); theNumber of Reflecting Optical Elements is Two, and Wavelength Limitationis Performed at the Time of Multi-Wavelength Transmission>>

[0085]FIG. 23 shows the optical structure and the optical path of aseventeenth embodiment. The seventeenth embodiment is characterized inthat three wavelengths are limited when light is transmitted by thesecond reflecting optical element (D2). Except this, the structure issimilar to that of the sixth embodiment (FIG. 8). That is, the first andthe second reflecting optical elements (D1, D2) are each a diffractionoptical element (HOE) having three different diffraction centerwavelengths (for example, red (R), green (G) and blue (B) correspondingto the three primary colors), and transmission to cut away a wavelengthregion on the longer wavelength side of each of the wavelengthdistributions of the reflected light beam (L3) corresponding to thediffraction center wavelengths is performed at the second reflectingoptical element (D2). FIGS. 24(A) and 24(B) show the change of thewavelength distribution.

[0086] The graphs in FIGS. 24(A) and 24(B) show the wavelengthdistributions of the light beams before and after the wavelengthlimitation on the longer wavelength side of each of the threewavelengths (the horizontal axis represents the wavelength, and thevertical axis represents the light intensity). As shown in FIG. 24(A), awavelength region (E, the hatched part on the right side of the brokenline) on the longer wavelength side of the wavelength distribution ofthe reflected light beam (L3) is cut away by the transmission at thesecond reflecting optical element (D2), so that the wavelength width isnarrower. Consequently, as shown in FIG. 24(B), a sharp wavelengthdistribution (L4) with a narrower wavelength width is obtained for thewavelength regions corresponding to the diffraction center wavelengths.

[0087] The first and the second reflecting optical elements (D1, D2) mayeach be one comprising one element (for example, an HOE formed bymultiple exposure) or may each be one comprising at least two elementslimiting the wavelengths which elements are superposed one on another orbonded together. In the structure designed for a multiplicity ofwavelengths like the present embodiment, a diffraction effect of highefficiency can be obtained when the one comprising a plurality ofelements superposed one on another or bonded together is used, althoughthe one formed by multiple exposure is easier to form. Moreover, thewavelength limitation may be performed at the time of reflection or atthe time of reflection and transmission like in the twelfth, thefifteenth and the sixteenth embodiments (FIGS. 15, 19 and 21).

[0088] By the first and the second reflecting optical elements (D1, D2)being diffraction optical elements (HOEs or the like) having at leasttwo diffraction center wavelengths and by performing at least once thetransmission or the reflection to cut away a wavelength region on thelonger wavelength side or the shorter wavelength side of each of thewavelength distributions, corresponding to the diffraction centerwavelengths, obtained by performing reflection or transmission at leastonce as described above, the wavelength distribution of the incidentlight beam (L0) can be changed to one with a narrow width (L4) for thewavelength regions corresponding the diffraction center wavelengths. Forexample, the light beams of the wavelengths corresponding to R, G and Bof an LED light source can be narrowed in wavelength, so that aninexpensive and thin color (RGB) band-pass filter can be obtained. Whenthe first and the second reflecting optical elements (D1, D2) compriseHOEs which are easy to form, the limitation of the wavelengths can beefficiently performed.

[0089] <<Embodiment of the Band-Pass Filter (FIGS. 25 and 26); theNumber of Reflecting Optical Elements is Two (with an Optical Power),and Wavelength Limitation is Performed at the Time of Transmission>>

[0090]FIG. 25 shows the optical structure and the optical path of aneighteenth embodiment. The eighteenth embodiment is characterized inthat the second reflecting optical element (D2) has a positive opticalpower. Except this, the structure is similar to that of the sixthembodiment (FIG. 6). In this band-pass filter, the incident light beam(L0) diverging from a light source such as an LED is transmitted by thefirst reflecting optical element (D1), the transmitted light beam (L1)is substantially vertically reflected by the positive optical power ofthe second reflecting optical element (D2), and the reflected light beam(L2) is substantially vertically reflected by the first reflectingoptical element (D1). The reflected light beam (L3) iswavelength-limited when substantially vertically transmitted by thesecond reflecting optical element (D2) and exits as a paralleltransmitted light beam (L4). FIGS. 26(A) and 26(B) show the change ofthe wavelength distribution.

[0091] The graphs in FIGS. 26(A) and 26(B) show the wavelengthdistribution of the light beam before and after the wavelengthlimitation (the horizontal axis represents the wavelength, and thevertical axis represents the light intensity). As shown in FIG. 26(A), awavelength region of the wavelength distribution of the transmittedlight beam (L1) is cut away by the transmission at the second reflectingoptical element (D2), so that the wavelength width is narrower.Consequently, as shown in FIG. 26(B), a sharp wavelength distribution(L4) with a narrow wavelength width is obtained.

[0092] While a positive optical power is provided to the secondreflecting optical element (D2), an optical power may be provided to thefirst reflecting optical element (D1). Moreover, either a positive or anegative optical power may be provided to both the first and the secondreflecting optical elements (D1, D2). By providing either a positive ora negative optical power to at least one of the first and the secondreflecting optical elements (D1, D2), a lens function (for example, acondenser lens function) can be added to the band-pass filter, so that amultifunction band-pass filter is obtained.

[0093] <<Embodiments of the Illuminating Optical System (FIGS. 27 and28); the Number of Reflecting Optical Elements is Two (with a LensFunction)>>

[0094]FIGS. 27 and 28 show the optical structures and the optical pathsof a nineteenth and a twentieth embodiment. The nineteenth embodiment ischaracterized in that the second reflecting optical element (D2) has apositive optical power. The twentieth embodiment is characterized inthat the first reflecting optical element (D1) has a negative opticalpower. In the illuminating optical systems of the nineteenth and thetwentieth embodiments, the optical path is set so that the incidentlight beam (L0) is transmitted by the first reflecting optical element(D1), the transmitted light beam (L1) is reflected by the secondreflecting optical element (D2), the reflected light beam (L2) isreflected by the first reflecting optical element (D1) and then, thereflected light beam (L3) is transmitted by the second reflectingoptical element (D2) so that the transmitted light beam (L4) exits.Because of the optical powers of the first and the second reflectingoptical elements (D1, D2) each comprising a diffraction optical element,the optical system functions as a positive lens (such as a condenserlens) and a negative lens as a whole. These basic structures are similarto those of the eighteenth embodiment (FIG. 25).

[0095] By at least one of the first and the second reflecting opticalelements (D1, D2) being a diffraction optical element and by bendinglight by the diffraction action when the light is reflected ortransmitted as described above, a compact, lightweight and simpleilluminating optical system can be easily obtained. The wavelengthlimitation function may be provided together with the lens function likein the eighteenth embodiment (FIG. 25). While it is desirable that thetwo reflecting optical elements (D1, D2) have different opticalcharacteristics, they may have the same optical characteristic. Theoptical surfaces of the reflecting optical elements (D1, D2) are notlimited to plane surfaces but may be curved surfaces. The reflectingoptical elements (D1, D2) may be disposed so as to be inclined withrespect to each other. The number of reflecting optical elements may bethree or more. Polarization may be used. Moreover, the first and thesecond reflecting optical elements (D1, D2) may each be one comprisingone element (for example, an HOE formed by multiple exposure) or mayeach be one comprising at least two elements superposed one on anotheror bonded together. The light beams of the wavelengths corresponding,for example, to R, G and B of an LED light source may be bent by usingas the reflecting optical elements (D1, D2) diffraction optical elementshaving at least two different diffraction center wavelengths.

[0096] <<Embodiments of the Illuminating Optical System (FIGS. 29 to31); the Number of Reflecting Optical Elements is Two (with a ColorMatching Function)>>

[0097] FIGS. 29 to 31 show the optical structures and the optical pathsof a twenty-first to a twenty-third embodiment. The twenty-firstembodiment is characterized in that the second reflecting opticalelement (D2) has an RGB color matching function. The twenty-secondembodiment is characterized in that the first and the second reflectingoptical elements (D1, D2) have an RGB color matching function. Thetwenty-third embodiment is characterized in that the first and thesecond reflecting optical elements (D1, D2) bonded to both surfaces of awedge prism (PR) have an RGB color matching function.

[0098] In the illuminating optical systems of the twenty-first to thetwenty-third embodiments, the optical path is set so that the incidentlight beams (L0) of R, G and B are transmitted by the first reflectingoptical element (D1), the transmitted light beams (L1) are reflected bythe second reflecting optical element (D2), the reflected light beams(L2) are reflected by the first reflecting optical element (D1) andthen, the reflected light beams (L3) are transmitted by the secondreflecting optical element (D2) so that the transmitted light beams (L4)exit. By the diffraction actions of the first and the second reflectingoptical elements (D1, D2) each comprising a diffraction optical element,color matching of R, G and B is performed. By this color matchingfunction, color matching can be performed by bending light so that thevirtual image positions of the illuminating light sources of R, G and Bsubstantially coincide with one another. Moreover, according to thetwenty-third embodiment in which color matching is performed by thefirst and the second reflecting optical elements (D1, D2) bonded to bothsurfaces of the wedge prism (PR), since the shift among the light beamscan be corrected by the chromatic dispersion action of the wedge prism(PR), color matching is more easily performed.

[0099] By at least one of the first and the second reflecting opticalelements (D1, D2) being a diffraction optical element and by performingcolor matching by bending light by the diffraction action when the lightis reflected or transmitted as described above, a compact, lightweightand simple illuminating optical system can be easily obtained. Thewavelength limitation function may be provided together with the colormatching function. While it is desirable that the two reflecting opticalelements (D1, D2) have different optical characteristics, they may havethe same optical characteristic. The optical surfaces of the reflectingoptical elements (D1, D2) are not limited to plane surfaces but may becurved surfaces. The reflecting optical elements (D1, D2) may bedisposed so as to be inclined with respect to each other. The number ofreflecting optical elements may be three or more. Polarization may beused. Moreover, the first and the second reflecting optical elements(D1, D2) may each be one comprising one element (for example, an HOEformed by multiple exposure) or may each be one comprising at least twoelements superposed one on another or bonded together. Color matchingmay be performed by bending the light beams of the wavelengthscorresponding, for example, to R, G and B of an LED light source byusing as the reflecting optical elements (D1, D2) diffraction opticalelements having at least two different diffraction center wavelengths.

[0100] <<Embodiment of the Illuminating Optical System (FIG. 32); theNumber of Reflecting Optical Elements is Two (with a Lens Function and aColor Matching Function)>>

[0101]FIG. 32 shows the optical structure and the optical path of thetwenty-fourth embodiment. The twenty-fourth embodiment is characterizedin that both the lens function of the nineteenth embodiment (FIG. 27)and the color matching function of the twenty-second embodiment (FIG.30) are provided. The combination of the lens function and the colormatching function is not limited to the one of the present embodiment,but the lens function of the twentieth embodiment (FIG. 28) may becombined or the color matching function of the twenty-first or thetwenty-third embodiment (FIG. 29 or 31) may be combined.

[0102] <<Embodiment of the Beam Shaping Optical System (FIGS. 33 to 35);the Number of Reflecting Optical Systems is Two>>

[0103]FIGS. 33 and 34 show optical cross-sectional structures and theoptical paths, in an x direction (minor axis direction) and in a ydirection (major axis direction), of a twenty-fifth embodiment,respectively. FIGS. 35(A) and 35(B) show beam shapes (PN1, PN2) beforeand after shaping, respectively. The twenty-fifth embodiment ischaracterized in that different optical powers are provided in the x andthe y directions. That is, the beam shaping optical system of thepresent embodiment is an anamorphic optical system having differentoptical powers in the x and the y directions, and the beam diameter isincreased in the x direction by the negative optical power and isreduced in the y direction in the positive optical power. Therefore, anelliptical beam (PN1) emanating from a laser light source (s, such as asemiconductor laser) as shown in FIG. 35(A) is shaped into a circularbeam (PN2) as shown in FIG. 35(B).

[0104] In the beam shaping optical system of the twenty-fifthembodiment, the optical path is set so that the incident light beam (L0)is transmitted by the first reflecting optical element (D1), thetransmitted light beam (L1) is reflected by the second reflectingoptical element (D2), the reflected light beam (L2) is reflected by thefirst reflecting optical element (D1) and then, the reflected light beam(L3) is transmitted by the second reflecting optical element (D2) sothat the transmitted light beam (L4) exits, and when light is reflectedby the reflecting optical elements (D1, D2) each comprising adiffraction optical element, the cross-sectional shape of the beam ischanged by the diffraction action. By at least one of the first and thesecond reflecting optical systems (D1, D2) being a diffraction opticalelement and by changing the cross-sectional shape of the beam by thediffraction action when the light is reflected or transmitted asdescribed above, a compact, lightweight and simple beam shaping opticalsystem can be easily obtained. The first and the second reflectingoptical elements (D1, D2) may each be one comprising one element (forexample, an HOE formed by multiple exposure) or may each be onecomprising at least two elements superposed one on another or bondedtogether (that is, one where the elements are disposed so that theoptical powers in the x and the y directions are in an anamorphicrelationship).

[0105] <<Embodiments of the Image Display Apparatus (FIGS. 36 to 40)>>

[0106] FIGS. 36 to 40 show the optical structures and the optical pathsof a twenty-sixth to a thirtieth embodiment. In FIGS. 36 to 40,reference number 1 represents an LED, reference number 2 represents acondenser lens for illumination, reference number 3 represents atransmissive LCD, reference number 4 represents a holographic magnifyingoptical system, reference number 5 represents the viewer's eye,reference number 6 represents a prism, and BP represents a band-passfilter (BP) corresponding to the first to the eighteenth embodiments.The LED (1) is an illuminating light source emitting light forilluminating the display screen (3 s) of the LCD (3). The condenser lens(2) is a collimator lens collimating the light from the LED (1). The LCD(3) is a transmissive spatially modulating element displayingtwo-dimensional images on the display screen (3 s). Since the LCD (3) isa nonradiative display element, the two-dimensional images are madeviewable by the display screen (3 s) being illuminated with theillumination light from the LED (1), and the holographic magnifyingoptical element (4), serving as an eyepiece optical system, projects thetwo-dimensional images onto the viewers eye so as to be magnified.

[0107] The twenty-sixth to the thirtieth embodiments are eachcharacterized by the position of the band-pass filter (BP) on theoptical path. In the twenty-sixth embodiment (FIG. 36), the band-passfilter (BP) is interposed between the condenser lens (2) and the LCD(3). In the twenty-seventh embodiment (FIG. 37), the band-pass filter(BP) is interposed between the LED (1) and the LCD (3). The band-passfilter (BP) used in the twenty-seventh embodiment (FIG. 37) has acondenser lens function, and corresponds to the illuminating opticalsystem of the nineteenth embodiment (FIG. 27). Therefore, the condenserlens (2) is unnecessary. In the twenty-eighth embodiment (FIG. 38), theband-pass filter (BP) is interposed between the LED (1) and thecondenser lens (2). In the twenty-ninth embodiment (FIG. 39), theband-pass filter (BP) is interposed between the LCD (3), and the prism(6) and the holographic magnifying optical element (4). In the thirtiethembodiment (FIG. 40), the band-pass filter (BP) is disposed on the prism(6) between the LCD (3) and the holographic magnifying optical element(4). At the first reflecting optical element (D1), reflection isperformed twice and transmission is performed once, and at the secondreflecting optical element (D2), reflection and transmission are eachperformed once. The band-pass effect is obtained by performing thewavelength limitation at one of the reflections and the transmissions.

[0108] By using the band-pass filter (BP) like in the twenty-sixth tothe thirtieth embodiments, a sharp wavelength distribution with a narrowwavelength width is obtained, so that high-quality images can bedisplayed with an inexpensive, compact, lightweight and simplestructure. Moreover, by using the band-pass filter (BP), the chromaticaberration of the diffracted light can be appropriately reduced by theholographic magnifying optical element (4).

[0109]FIG. 41 shows an example using the band-pass filter (BP) of thepresent invention for an eyeglass-type image display apparatus. In FIG.41, reference number 4 represents a holographic magnifying opticalelement, reference number 6 represents a prism, reference number 7represents a cable, reference number 8 represents a bridge, referencenumbers 8R and 8L represent nose pads, reference numbers 9R and 9Lrepresent lenses, reference number 10 represents a display, referencenumbers 11R and 11L represent temples, and reference number 14represents a case. The prism (6) is imbedded in part of the lens (9R)for the right eye, and the display (10) displaying images is attached toan upper part of the prism (6). To the display (10), a power supplyportion and the cable (7) for signal supply are connected. The display(10) is covered with the case 14, and the prism (6) is sandwiched by theprism (6). The LED (1), the condenser lens (2); the LCD (3) and the likeare provided in the case (14). With this structure, a compact andlightweight image display apparatus can be structured.

[0110] The above-described first to thirtieth embodiments includeinventions (j1 to j22) having the following structure:

[0111] As described above, according to the band-pass filter of thepresent invention, a sharp wavelength distribution is obtained and theband-pass filter is reduced in size and weight. According to theilluminating optical system of the embodiments, since light is bent bythe diffraction action, the condenser lens function and the colormatching function are obtained with a compact structure. According tothe beam shaping optical system of the present invention, since thecross-sectional shape of the beam is changed by the diffraction action,an excellent beam shaping function is obtained with a compact structure.By using the band-pass filter and the illuminating optical system of theembodiments, a compact and lightweight image display apparatus isrealized that is capable of displaying high-quality images with aninexpensive and simple structure.

[0112] Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

What is claimed is:
 1. A band-pass filter comprising a reflecting optical element having a fixed reflection angle selection region for wavelengths of a given region, and has an optical path such that reflection is performed at least once and transmission is performed at least once at the reflecting optical element.
 2. A band-pass filter as claimed in claim 1, wherein a wavelength of a light beam reflected by the reflecting optical element is narrowed by being cut when the light beam is transmitted.
 3. A band-pass filter as claimed in claim 1, wherein the reflecting optical element is a volume-phase holographic optical element, a multilayer film or a multilayer filter.
 4. A band-pass filter comprising: at least two reflecting optical elements each having a fixed reflection angle selection region for wavelengths of a given region, and has an optical path such that the optical wave is reflected at least once by at least one of the reflecting optical elements and either reflection or transmission is performed at another of the reflecting optical elements.
 5. A band-pass filter as claimed in claim 4, wherein a wavelength of a light beam reflected by the reflecting optical element is narrowed by being cut when the light beam is transmitted.
 6. A band-pass filter as claimed in claim 4, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected by another of the reflecting optical elements.
 7. A band-pass filter as claimed in claim 4, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected and transmitted by another of the reflecting optical elements.
 8. A band-pass filter as claimed in claim 4, wherein a wavelength of an incident light beam is cut when the light beam is reflected and transmitted by one of the reflecting optical elements, and the wavelength is narrowed by being cut when the light beam is transmitted and reflected by another of the reflecting optical elements.
 9. A band-pass filter as claimed in claim 4, wherein the reflecting optical elements are volume-phase holographic optical elements, multilayer films or multilayer filters.
 10. A band-pass filter that comprises at least two reflecting optical elements each having a fixed reflection angle selection region for wavelengths of a given region, and has an optical path such that the optical wave is reflected at least once and transmitted at least once by at least one of the reflecting optical elements and either reflection or transmission is performed by another of the reflecting optical elements.
 11. A band-pass filter as claimed in claim 10, wherein a wavelength of a light beam reflected by the reflecting optical element is narrowed by being cut when the light beam is transmitted.
 12. A band-pass filter as claimed in claim 10, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected by another of the reflecting optical elements.
 13. A band-pass filter as claimed in claim 10, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected and transmitted by another of the reflecting optical elements.
 14. A band-pass filter as claimed in claim 10, wherein a wavelength of an incident light beam is cut when the light beam is reflected and transmitted by one of the reflecting optical elements, and the wavelength is narrowed by being cut when the light beam is reflected and transmitted by another of the reflecting optical elements.
 15. A band-pass filter as claimed in claim 10, wherein the reflecting optical elements are volume-phase holographic optical elements, multilayer films or multilayer filters.
 16. A band-pass filter that comprises at least two reflecting optical elements each having a fixed reflection angle selection region for wavelengths of a given region, and has an optical path such that reflection is performed once and transmission is performed once by at least two of the reflecting optical elements.
 17. A band-pass filter as claimed in claim 16, wherein a wavelength of a light beam reflected by the reflecting optical element is narrowed by being cut when the light beam is transmitted.
 18. A band-pass filter as claimed in claim 16, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected by another of the reflecting optical elements.
 19. A band-pass filter as claimed in claim 16, wherein a wavelength of a light beam reflected by one of the reflecting optical elements is narrowed by being cut when the light beam is reflected and transmitted by another of the reflecting optical elements.
 20. A band-pass filter as claimed in claim 16, wherein a wavelength of an incident light beam is cut when the light beam is reflected and transmitted by one of the reflecting optical elements, and the wavelength is narrowed by being cut when the light beam is reflected and transmitted by another of the reflecting optical elements.
 21. A band-pass filter as claimed in claim 16, wherein the reflecting optical elements are volume-phase holographic optical elements, multilayer films or multilayer filters.
 22. A band-pass filter as claimed in claim 16, wherein a volume-phase holographic optical element has at least two different diffraction center wavelengths, and limits at least two wavelengths. 